1. Field of the Invention
The present invention relates generally to ophthalmic testing instruments and, more particularly, to an improved device adapted both to measure the visual acuity of a patient and to function as an optical prescription calculator.
2. The Prior Art
The determination of visual acuity is an essential part of every eye examination. During the course of such an examination, acuity may be measured repeatedly to ascertain the resolution of each eye independently and both eyes together. The examination may also consist of independent and combined testing of the eyes with the aid of corrective lenses. In fact, the repeated determination of acuity forms an essential part of the process of refracting or determining the optimal corrective lenses to alleviate the effects of ametropia as well as a means for assessing the progress of ocular pathology.
Originally, clinical methods for measuring visual acuity involved the use of wall charts containing a fixed array of Snellen letters, Tumbling E targets or other accepted acuity targets. The patient ordinarily viewed the charts from a fixed distance (usually 20 feet). With the advent of ophthalmic devices, clinical testing methods have become more sophisticated. One device involves the projection of targets onto screens placed at a fixed distance by means of an optical system. See U.S. Pat. No. 3,517,988. Another device employs an optical system for measuring the acuity of a low-vision patient. See U.S. Pat. No. 3,639,042. Another discloses a method for testing the glare susceptibility of a patient. See U.S. Pat. No. 3,684,355. A more sophisticated device teaches the use of a computer controlled method for the automatic visual acuity determination of a patient. See U.S. Pat. No. 3,705,003. Another automated visual examination device, one that includes a feature of mapping blind spot locations, is shown and described in U.S. Pat. No. 3,737,217. A further automated visual examination device, including fixation monitoring compensation, is described in U.S. Pat. No. 3,883,235. Another device for determining visual acuity that presents a plurality of targets seriatim is disclosed in U.S. Pat. No. 3,905,688. An automated refraction apparatus and method are disclosed in U.S. Pat. No. 3,969,020. A further automated visual examination apparatus, including mapping blind spot locations, is taught in Reissue U.S. Pat. No. Re 28,921. A device for generating and displaying visual acuity targets in a clinical setting is shown and described in U.S. Pat. No. 4,239,351. The patent discloses first and second electro-optical display devices for displaying visual acuity targets, one to be viewed by a patient and the other by an examiner. The inventions disclosed and claimed herein are improvements to U.S. Pat. No. 4,239,351.
As regards the productivity of the eye examiner, the time required to instruct the patient regarding the target he is to view, or the time the clinician requires to find, adjust and align the required target in the case of projection devices becomes increasingly important, especially with heavy patient loads. Consequently, examination time has become a critical and sparse resource for most eye examiners, one that must be husbanded carefully. There is thus room left for improvements.